In endeavors such as the exploration or extraction of subterranean resources such as oil, gas, and geothermal energy, it is common to form boreholes in the earth. To form such a borehole 111, a specialized drill bit 112 may be suspended from a derrick 113 by a drill string 114 as shown in FIG. 1. This drill string 114 may be formed from a plurality of drill pipe sections 115 fastened together end-to-end. As the drill bit 112 is rotated, either at the derrick 113 or by a downhole motor, it may engage and degrade a subterranean formation 116 to form a borehole 111 therethrough. Drilling fluid may be passed along the drill string 114, through each of the drill pipe sections 115, and expelled at the drill bit 112 to cool and lubricate the drill bit 112 as well as carry loose debris to a surface of the borehole 111 through an annulus surrounding the drill string 114.
Various electronic devices, such as sensors, receivers, communicators or other tools, may be disposed along the drill string or at the drill bit. To power such devices, it is known to generate electrical power downhole by converting kinetic energy from the flowing drilling fluid by means of a generator. One example of such a downhole generator is described in U.S. Pat. No. 8,957,538 to Inman et al. as comprising a turbine located on the axis of a drill pipe, which has outwardly projecting rotor vanes, mounted on a mud-lubricated bearing system to extract energy from the flow. The turbine transmits its mechanical energy via a central shaft to an on-axis electrical generator which houses magnets and coils.
One limitation of this on-axis arrangement, as identified by Inman, is the difficulty of passing devices through the drill string past the generator. Passing devices through the drill string may be desirable when performing surveys, maintenance and/or fishing operations. To address this problem, Inman provides a detachable section that can be retrieved from the downhole drilling environment to leave an axially-located through bore without removing the entire drill string.
The turbine described by Inman is known as an axial turbine because the fluid turning the turbine flows parallel to the turbine's axis of rotation. An example of an axial turbine 220 is shown in FIG. 2 connected to a rotor 221 portion of a generator 222. Both axial turbine 220 and rotor 221 may be disposed within and coaxial with a section of a drill pipe 215. Drilling fluid 223 flowing through the drill pipe 215 may engage a plurality of vanes 224 disposed about the axial turbine 220 causing both axial turbine 220 and rotor 221 to rotate on a fluid-lubricated bearing system 225. In the embodiment shown, the rotor 221 comprises a plurality of magnets 226 disposed about the rotor 221. Movement of the magnets 226 may induce electrical current in coils of wire 227 wound around poles 228 of a stator 229.
It may be typical in downhole applications employing an axial turbine to pass around 800 gallons/minute (3.028 m3/min) of drilling fluid past such a turbine. As the drilling fluid rotates the axial turbine, it may experience a pressure drop of approximately 5 pounds/square inch (34.47 kPa). Requiring such a large amount of drilling fluid to rotate a downhole turbine may limit a drilling operator's ability to control other drilling operations that may also require a certain amount of drilling fluid.
A need therefore exists for a downhole turbine that requires less fluid flow to operate. An additional need exists for a downhole turbine that does not require retrieving a detachable section in order to pass devices through a drill string.